1. Field of the Invention
The present invention relates to a so-called back shooting-type ink jet printing head in which droplets are ejected in a direction opposite to a direction along which bubbles grow.
2. Description of the Related Art
An ink jet printing head mounted on an ink jet printing apparatus is structured so that minute ink droplets are ejected through minute ejection ports to perform a printing operation onto a print medium. A printing head using an electrothermal transducing element (heater) as an ink ejection energy generation means causes ink surrounding the heater to be heated within a short time in order to eject ink droplets. Bubbles are generated in ink that is filled in the interior of a liquid chamber of the printing head. Then, the generated bubbles are caused to expand to apply a pressure to the ink filled in the liquid chamber. As a result, the ink in the vicinity of the ejection port is caused to pass an ejection port and is ejected in the form of droplets. Methods for ejecting ink by a printing head may be classified depending on the relation between a bubble growth direction and an ink ejected direction. According to the back shooting method as an ink ejecting method, a direction along which bubbles grow is opposite to a direction along which droplets are ejected.
Such ink jet printing apparatus of the back shooting type is proposed by for example Japanese Patent Laid-Open No. 2004-351931. Japanese Patent Laid-Open No. 2004-351931 discloses a plate including an ejection port that includes a relatively-thick heat diffusion layer that is a layer at the surface opposed to a print medium. Thus, the ejection port has a sufficient length so that accuracy of ink ejection through the ejection port is improved.
FIG. 11 shows an example of a conventional printing head using the back shooting method. FIG. 11 is a cross-sectional view illustrating the structure of a printing head when Tape Automated Bonding (TAB) is used to install an electrical wiring portion on a substrate. At the surface of a silicon substrate 100 in the printing head, a liquid path having a predetermined depth forms a liquid chamber 106 when the silicon substrate 100 is joined with an orifice plate 130 (which will be described later). The liquid chamber 106 is filled with ink to be ejected through the printing head. The back face side includes an ink supply port 102 for supplying ink to the liquid chamber 106.
The upper part of the silicon substrate 100 is joined with an orifice plate 130. The orifice plate 130 is joined with the silicon substrate to form an upper wall of the liquid chamber 106. This orifice plate 130 includes a plurality of ejection ports 104 for ejecting ink from the liquid chamber 106. The ejection ports are arranged in two columns so as to penetrate the orifice plate 130 in the thickness direction. The orifice plate 130 consists of a plurality of layers layered on the silicon substrate 100. Among these layers, heaters 108 are arranged. The heaters 108 are electrically connected by a conductor 112 to a bonding pad 101.
The bonding pad 101 is electrically connected via a bump 121 to an inner lead 120 formed in the printing apparatus-side by the TAB. Such an electrical connection part is covered by sealant 124 in order to protect this part from an external environment. The sealant 124 is formed to have a convex shape at the periphery of the bonding pad 101. Thus, the sealant 124 protrudes from an ejection port formation surface of the orifice plate 130. A support base 123 is the support base of the printing head.
The following section will describe a mechanism through which the printing head using the back shooting method as described above is used to eject ink through the ejection port 104.
First, pulsed current is applied to the heater 108 via the conductor 112 while the liquid chamber 106 and the ejection port 104 are being filled with ink. The electric energy is transduced to thermal energy and the heater 108 generates heat. The heat generated by the heater 108 is used to heat the ink on the heater 108. When the temperature of heated ink exceeds the boiling point, the ink on the heater 108 boils to generate bubbles. Continuous heat supply causes the generated bubbles to grow from the heater 108 and toward the lower side in FIG. 11. As a result, a part of the ink surrounding the ejection port 104 is extruded from the ejection port 104 to the upper side in FIG. 11. In this manner, the ink stored in the liquid chamber 106 is ejected in the form of droplets in a direction opposite to a direction along which bubbles grow (a direction toward the print medium).
When the current applied to the heater 108 is blocked, bubbles contract and finally disappear. With the contraction of bubbles, ink is supplied from the ink supply port 102 via an ink flow path 105 into the liquid chamber 106 to fill ink in the liquid chamber 106 again. When the ink refill process is completed to return to an initial state, the steps as described above are repeated. In this manner, ink is continuously ejected through the ejection port 104.
In order to maintain a high-quality printing by the printing head as described above, it is required that a high accuracy of ejection is secured during the ejection of droplets. In order to secure a high ejection accuracy of droplets, it is effective to minimize the distance between an ejection port face and a print medium.
In the case of the conventional back shooting-type ink jet printing head as shown in FIG. 11, however, the electrical wiring portion positioned at the obverse face of the orifice plate 130 is covered by the sealant 124, and the sealant 124 protrudes closer to the print medium-side (the upper side in FIG. 11) than the ejection port face of the printing head.
Due to the structure as described above in which the sealant 124 protrudes closer to the print medium than the ejection port face, the ejection port face of the printing head is prevented from approaching the print medium. As a result, the distance between the ejection port face of the printing head and the print medium cannot be sufficiently reduced, making it difficult to keep the ink ejection accuracy high.
Furthermore, in the case of the conventional back shooting-type printing head shown in FIG. 11, the silicon substrate 100 includes the ink supply port 102 formed so that the flow path has a narrower width toward the ejection port. From the ink supply port 102, the liquid chamber 106 is formed to extend toward the ejection port. Thus, the silicon substrate 100 includes therein a space having a complicated shape, thus possibly causing the time for processing this space to be long. This may cause an increased manufacture cost of the printing head.